The production of structures in the micrometer and/or nanometer range is carried out to an increasing extent using imprint lithography. In recent years, imprint lithography has always been able to hold its own compared to photolithography. The merits of imprint lithography primarily lie in the possibility of the production of structures in the nanometer range, whose production would not be at all feasible by means of photolithography or would be extremely expensive. Now, patterns in the nanometer and/or micrometer range can be produced with high precision and accuracy in a mass process on the surfaces of substrates. Although the merits of imprint lithography lie primarily in the production of structures in the nanometer range, there is also a very large area of application in the micrometer range for the imprint technologies, most particularly in the lens-embossing technique.
The lens-embossing techniques can be used in embossing techniques for the production of so-called monolithic lens wafers. The production process is called monolithic lens molding (MLM). With this technique, several lenses are produced as part of one and the same substrate. The lenses are not separated from one another and therefore also are not necessarily dependent upon a carrier substrate, although a connection to a carrier substrate or any other second substrate can be produced.
In another embossing technique, the lenses are embossed at the same time, but are not connected to one another by means of the embossing material. For quick and efficient further processing, this embossing is carried out in most cases on a carrier substrate. Drops of the embossing material, in most cases by a dispensing system that moves relative to the rigid carrier substrate, are deposited on the carrier substrate. Then, an approximate relative movement between the embossing die and the carrier substrate occurs. Preferably, only the die is brought closer relative to the static carrier substrate. In this case, each individual lens is to be as symmetric as possible. This is primarily then ensured when the drops of the embossing material were positioned exactly below the embossing structures. In addition, during the embossing process, it must be ensured that the embossing die (and thus the embossing structures) does not move relative to the drops of embossing material in an X-direction and/or a Y-direction during the approach in the Z-direction. The drops of the embossing material can be pressed and formed partially to the side during the embossing process, but in general, they no longer leave their position on the carrier substrate since the prevailing adhesion between carrier substrate and embossing material is too great for this purpose.
In a third embossing technique, a so-called step-and-repeat embossing die is used. The step-and-repeat embossing die is in this case smaller than the substrate, on which the lenses are to be embossed. In general, the step-and-repeat embossing die even has only one single lens shape and can thus emboss, if any, only one individual lens for each embossing step. In this embossing technique, preferably in turn drops of the embossing material are distributed on a substrate. Then, the step-and-repeat embossing die starts each drop individually and performs the embossing. In special cases, the deposition of a full-surface layer on the embossing material is also conceivable, which embossing material is then structured by a step-and-repeat embossing die. Some step-and-repeat embossing dies have several lens shapes of the same or different shape and thus are between the full-surface and the pure step-and-repeat embossing dies. Correspondingly, they can simultaneously emboss several lenses for each embossing step.
The quality of an embossed product, for example a lens wafer, or corresponding single lenses on a carrier substrate, therefore very greatly depends on the interaction between the die and the embossing material. Thus, during the dispensing of the embossing material and/or the embossing process, defects such as gas bubbles, differences in thickness along the surface, unevenness in density, runny or asymmetric embossing material, etc., can develop.
Because of the highly-viscous embossing material, gas bubbles are not specifically produced during the embossing process in most cases but rather are already located in the embossing material, for example by a false filling of the dispensing system. Nevertheless, such gas bubbles can sometimes even develop during the dispensing itself.
The differences in thickness along one surface are in most cases a result of an existing wedge error and can be avoided to a very large extent by a correct positioning between embossing die and carrier substrate.
Some unevenness in density is likely of a chemical nature and caused to a lesser extent by the dispensing system. Very possibly, however, the cross-linking of a polymer at various sites can occur at various levels of strength during a hardening process and can lead to a corresponding unevenness in density.
Asymmetric embossing materials can primarily occur in a dispensing of drops. In this case, the embossing material is distributed in single drops over a surface and is not completely symmetric to the embossing structures deforming them, so that during or after the embossing, an asymmetric single lens is produced.
One of the greatest problems with the current embossing technology consists primarily in the incomplete distribution or filling of the embossing structures of the embossing die by the embossing material. During the embossing process, the die presses the embossing material radially outward. At the same time, the embossing structures are filled with the embossing material. Ambient gases can be enclosed between the embossing material and the surface of the embossing structures of the embossing die by this process. These ambient gases produce corresponding bubbles and thus destroy the homogeneity of the material. This can have fatal effects primarily in the case of optical products such as lenses. Lenses with such effects would have lens defects, in particular chromatic and spherical aberrations. It would be conceivable to allow a corresponding embossing process to take place in a vacuum. To this end, the corresponding chamber has to be evacuated before each embossing process. After successful embossing, the chamber would again be ventilated. These processes are correspondingly time-intensive and therefore very expensive.
Another problem, which primarily occurs in the dispensing of drops of embossing material for the production of single lenses distributed on a carrier substrate, is the symmetry of individual lenses. A poorly-positioned drop of embossing material on the carrier substrate and/or a poor approach of the embossing die and therefore the embossing structures, relative to the drops of embossing material, lead to asymmetric lens shapes.
The flowing of the embossing material, which can be attributed in particular to the adhesion property between the embossing material and the corresponding surface, represents another problem.
It is therefore the object of this invention to solve one or more of the above-mentioned technical problems with a method and/or a device according to the subsequent description.
This object is achieved with the features of the independent claim(s). Advantageous further developments of the invention are indicated in the subclaims. All combinations that include at least two of the features indicated in the specification, the claims and/or the figures also fall within the scope of the invention. In the indicated ranges of values, values as boundary values that lie within the above-mentioned limits are also to be considered as disclosed and can be claimed in any combination.